Distribution System Planning and Design

Electrical Distribution System
Name of the Faculty : Adithya Ballaji
School of Electrical and Electronics Engineering
Unit 1: Distribution System Planning and Design
School of Electrical and Electronics Engineering, REVA
University

Syllabus
• Introduction, Factors affecting system planning, present planning
techniques, planning models, Sub-transmission and substation design.
Sub-transmission networks configurations, Substation bus schemes,
Distribution substations ratings, Service areas calculations, and
Substation application curves, future trends in planning, systems
approach, and Distribution automation.
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Introduction
• In general, the definition of an electric power system includes a generating, a transmission, and a
distribution system.
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Distribution System Layout
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• Sub-transmission line carry
voltages from major TLS.
• Mainly from 34.5 kv to 69 kv, this
power is sent to regional distribution
Substation, sometimes voltage is
Tapped along the way for use in
Industrial or large commercial
operations

SLD of GTD
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Distribution System
“The part of power system which distributes electrical power for local use is known as
DISTRIBUTION SYSTEM.”
• This system is the electrical system between the substation fed by the transmission system and
consumer meter.
• Distribution line generally consist of
1. Feeders
2. Distributers
3. Service mains
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Feeders
• A Feeder is conductor which connects the substation to the area where power is
to be distributed .
• Feeder are used to feed the electrical power from the generating station to the substation
• No tapings are taken from the feeder
• So the current in it remains the same throughout
• Main consideration in the design of feeder is the Current carrying capacity.
Distributer
• A distributer is a conductor from which tapings are taken from pole mounted transformer to the
consumer
• The current through a distributer is not constant because tapings are taken at various places along
its length
• Voltage drop is main consideration
• Limit of variation is 6% of rated at consumer
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Service mains
• A service mains is a generally a small cable which connects the distributer to the
consumer ‘s meter.
The connecting links between the distributor and the consumer terminals.
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Distribution system
Distribution system is a part of power system, existing between distribution substations and
consumers.
It is further classified on the basis of voltage
Primary distribution system- 11 KV or 6.6 KV or 3.3 KV
Secondary distribution system- 415 V or 230 V
Classification Of Distribution System: It can be classified under different considerations as; 1. Type
Of Current: a) AC Distribution System b) DC Distribution System
2. Type Of Construction: a) Overhead System b) Underground System
3. Type Of Service: a) General Lighting & Power b) Industrial Power c) Railway d) Streetlight etc
4. Number Of Wires: a) Two Wire b) Three Wire c) Four Wire
5. Scheme Of Connection: a) Radial Distribution System b) Ring or Loop Distribution System c)
Interconnected Distribution System
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AC distribution
• AC distribution system is the electrical system between the step-down
substation fed by the transmission system and the consumers’ meters.
• The AC distribution system is classified into
• Primary distribution system
• Secondary distribution system.
• Why Study of Electrical Distribution System is Necessary?
• In recent years the investment trends show that the percentage of electrical plants in service by the
production, i.e, GTD major investment as been in generation followed closely in second by
distribution.
• The major expenses for O & M has been in production sector followed by distribution sector
• The economic importance of the distribution system is very high, and the amount of investment
involved needs careful planning, design, construction and operation
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Distribution System Planning
Distribution System planning is essential to ensure that the growing demand for
electricity can be satisfied by addition of DS which are technically adequate and also
economical.
The objective of distribution system planning is to assure that the growing demand for electricity, in
terms of increasing growth rates and high load demand, can be satisfied in an optimum way by
additional distribution systems
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First Distribution system planners must determine the load
magnitude and its geographic location. Then the DS must be
placed and sized in such a way as to serve the load at maximum efficiency,
being cost effectiveness by minimizing feeder losses and construction costs,
while considering the constraints of service reliability.

Factors Effecting DSP
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Demand of ever increasing power capacity, Higher distribution voltages, automation
and special control are some of the parameter which effect the DSP.
In general the factors effecting may include scarcity in availability of land in urban areas,
ecological considerations and limitation on fuel choices, investment etc.
• Load Forecasting
• The load demand growth of geographic area is the most important factor which influences the expansion
Of DS.
• Thus the forecasting of load increase and the system reaction to these increases is the most important in
planning process.
• Load forecasting is done in two ways based on time, long range forecasting for 15-20 years and short
range for 5 years.
• These forecasting would predict the future loads in details and the final loads are predicted in kVA

Factors affecting DSP
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Substation Expansion
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• The planner makes the decision based on tangible and intangible information.
• For example the forecasted load, load demand and load growth may require a substation expansion
or new substation.
Substation site selection
• The distance from the load centers and from the existing sub-transmission lines as well as other
limitations, such as availability of land, its cost, and land use regulations, is important.
• The substation siting process can be described as a screening procedure through which all possible
locations for a site are passed

Substation Site Selection
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• Service region is the area
Under Evaluation
• Candidate areas are divided
Into three groups:
1. Sites which are unsuitable
2. Sites which are initially
selected but rejected after
panning process.
3. Sites which needs to be
studied in detail

Other Factors
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• Once the loads are predicted and assigned, then other factors are considered
like, primary voltage selection, feeder route selection, number of feeders,
conductor size selection and total cost are taken into account for final
planning process.

Present Planning techniques
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• Computer based : many electric distribution system planners in the industry utilize computer
programs
• Such as load flow programs, radial or loop load flow programs, short-circuit and fault-current
calculation programs, voltage drop calculation programs, and total system impedance calculation
programs, as well as other tools such as load forecasting, voltage regulation, regulator setting,
capacitor planning, reliability, and optimal siting and sizing algorithms.
• The computers do perform calculations more expeditiously than other methods and free the
distribution engineer from detailed work. The engineer can then spend time reviewing results of the
calculations, rather than actually making them.

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Typical Distribution System
Planning Process

Distribution System Planning Models
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• In general, distribution system planning has a complex procedure due to a
large number of variables involved and mathematical requirements.
• Therefore, mathematical models are developed to represent the system and can be employed by
distribution system planners to investigate and determine optimum expansion patterns or
alternatives by selecting the following models:
1. Optimum substation locations
2. Optimum substation expansions
3. Optimum substation transformer sizes
4. Optimum load transfers between substations and demand centers
5. Optimum feeder routes and sizes to supply the given loads subject to numerous constraints to
minimize the present worth of the total costs involved

Sub-transmission System
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• The sub-transmission system is defined as the circuits which deliver energy from the transmission
system to the primary distribution system.
• But usually the sub-transmission system are supplied from the transmission sub-stations and still
referred to as sub-transmission.
• Earlier many of the sub-transmission system were the transmission lines, but sue the increase in the
load growth and demand for more power led to the reduction of voltage from 220kV to 33kV
which are found in the sub-transmission lines.
• Mainly the distribution system are considered to be consisting four main components : Its sub-
transmission system, the substation itself, the feeder system and the consumers.
• The distribution substation reduces the voltage to the lower primary voltage in the range of 22 to
11kV for local distribution.
• The local transformer reduces the voltages to 430/415V or 230/240V with a variation of +/- 6
percent at the consumer end.

Sub-transmission network configuration
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• Radial distribution system

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• Radial distribution system
• This system is used only when substation or generating station is located at the center of the
consumers.
• In this system, different feeders radiate from a substation or a generating station and feed the
distributors at one end.
• Thus, the main characteristic of a radial distribution system is that the power flow is in only one
direction.
• Although this system is simplest and least expensive, it is not highly reliable.
• A major drawback of a radial distribution system is, a fault in the feeder will result in supply
failure to associated consumers as there won't be any alternative feeder to feed distributors.

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• Ring main distribution system

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• Ring main distribution system
• Here, each distribution transformer is fed with two feeders but in different paths.
• The feeders in this system form a loop which starts from the substation bus-bars, runs through the
load area feeding distribution transformers and returns to the substation bus-bars.
• Ring main distribution system is the most preferred due to its following advantages.
Advantages of ring main distribution system
• There are fewer voltage fluctuations at consumer's terminal.
• The system is very reliable as each distribution transformer is fed with two feeders. That means, in
the event of a fault in any section of the feeder, the continuity of the supply is ensured from the
alternative path.

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• Interconnected distribution system

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• Interconnected distribution system
• When a ring main feeder is energized by two or more substations or generating stations, it is called
as an interconnected distribution system.
• This system ensures reliability in an event of transmission failure.
• Also, any area fed from one generating stations during peak load hours can be fed from the other
generating station or substation for meeting power requirements from increased load.

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• Parallel feeders distribution system

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• Parallel feeders distribution system
• The disadvantage of a radial system can be minimized by introducing parallel feeders.
• The initial cost of this system is much more as the number of feeders is doubled.
• Such system may be used where reliability of the supply is important or for load sharing where the
load is higher

Substation Bus Schemes
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• The electrical substation is a junction point where two or more transmission lines terminate.
• In reality, most EHV and HV substations can be the point where more than half a dozen of lines
terminate.
• In many large transmission substations, the total numbers of lines terminating exceeds one or two
dozen.
• A substation bus scheme is the arrangement of overhead bus bar and associated switching
equipment (circuit breakers and isolators) in a substation.
• The operational flexibility and reliability of the substation greatly depends upon the bus scheme.
• Types of bus scheme:
• Single Bus
• Main Bus and Transfer Bus
• Double Bus and Double Breaker
• Ring or Mesh Bus
• Break and Half

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Single Bus Configuration

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• As the name implies, the single bus substation configuration consists of all circuits connected to a
main bus.
• A fault on the bus or between the bus and circuit breaker will result in an outage of the entire bus
or substation.
• Failure of a single circuit breaker will also result in an outage of the entire bus.
• Maintenance of any circuit breaker requires shutdown of the corresponding circuit/line and
maintenance of the bus requires a complete shutdown of the bus.
• A bypass switch across the breaker should be used for maintenance of the corresponding breaker.
Circuit protection is disabled in this case.

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Main and Transfer Bus Configuration

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• In this arrangement one or more busses is added to the single bus substation scheme.
• One or more circuit breakers may be used in this arrangement to make connections between the
main and transfer bus.
• For maintenance of a circuit breaker, the transfer bus is energized by closing the isolator switches
to the transfer bus, then the circuit breaker to be maintained is opened and isolated on both sides.

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Double Bus Double Breaker

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• This configuration utilizes two buses and two breakers per circuit.
• Both buses are normally energized and any circuit can be removed for maintenance without an
outage on the corresponding circuit.
• Failure of one of the two buses will not interrupt a circuit because all of the circuits can be fed
from the remaining bus and isolating the failed bus.
• Substations with the double bus double breaker arrangement require twice the equipment as the
single bus scheme but are highly reliable.
• Load balancing between buses can be achieved by shifting circuits from one bus to the other.
• This scheme is typically found in EHV transmission substations or generating stations.

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Ring Bus Configuration

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• In the ring bus configuration, as the name implies, the circuit breakers are connected to form a ring,
with isolators on both sides of each breaker.
• Each circuit is fed from both sides.
• This scheme has good operational flexibility and high reliability, any of the circuit breakers can be
opened and isolated for maintenance without interruption of service.

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Breaker and Half Configuration

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• When expansion of the substation is required to accommodate more circuits, can easily be
expanded to the One and Half breaker configuration.
• This configuration uses two main buses, both of which are normally energized with three breakers
connected between the buses.
• In this bus configuration, three breakers are required for every two circuits - hence the "one and
half" name.
• Think of it as, to control one circuit requires one full and a half breaker.
• The middle breaker is shared by both circuits, similar to a ring bus scheme where each circuit is fed
from both sides.
• Any circuit breaker can be isolated and removed for maintenance purposes without interrupting
supply to any of the other circuits.
• Additionally, one of the two main busses can be removed for maintenance without interruption of
service to any of the other circuits.

Substation Bus Schemes - Comparison
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Type of Bus Scheme Advantages Disadvantages
Single bus 1. Lowest cost 1. Failure of bus or any circuit
breaker results in shutdown of
entire substation
2. Difficult to do any
maintenance
3. Bus cannot be extended
without completely de-
energizing substation
4. Can be used only where loads
can be interrupted or have other
supply arrangements

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Main-and-transfer 1. Low initial cost.
2. Any breaker can be taken out
of service for maintenance.
1. Requires one extra breaker
for the bus tie.
2. Switching is somewhat
complicated when maintaining a
breaker.

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Double bus–double breaker 1. Each circuit has two
dedicated breakers
2. Has flexibility in permitting
feeder circuits to be connected
to either bus
3. Any breaker can be taken out
of service for maintenance
4. High reliability
1. Most expensive
2. Would lose half the circuits
for breaker failure if circuits are
not connected to both buses

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Ring bus 1. Low initial cost
2. Flexible operation for breaker
maintenance
3. Any breaker can be removed
for maintenance without
interrupting load
4. Requires only one breaker per
circuit
5. Does not use main bus
6. Each circuit is fed by two
breakers
1. If a fault occurs during a
breaker maintenance period, the
ring can be separated into two
sections
2. Breaker failure during a fault
on one of the circuits causes
loss of one additional circuit

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Breaker-and-a-half l. Most flexible operation
2. High reliability
3. Breaker failure of bus side
breakers removes only one
circuit from service
4. Either main bus can be taken
out of service at any time for
maintenance
5. Bus failure does not remove
any feeder circuits from service
1. 1½ breakers per circuit
2. High Cost

Rating of Distribution Substation
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• Substations receive Electric energy from Generating stations or other Substations via long
Transmission lines with 110/230/400 kV system voltage.
• Substations are equipped with Transformers to step down the voltage to 11/33/66 kV to feed HT
distribution lines.
• Which is expressed in kilovolt-amperes (kVA) or megavolts- amperes (MVA), and indicates the
amount of power that can be transferred through the transformer. Distribution substation
transformers are typically in the range of 3 kVA to 25 MVA.
• The MVA capacity of these Transformers is called as the rating of the Substation.

Bus-bars or Buses
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• Bus-bars (also called buses) can be found throughout the entire power system, from generation to
industrial plants to electrical distribution boards. Bus-bars are used to carry large current and to
distribute current to multiple circuits within switchgear or equipment.
• Plug-in devices with circuit breakers or fusible switches may be
installed and wired.
• Originally, bus-bars consisted of uncovered copper conductors
supported on insulators, such as porcelain, mounted within a
non-ventilated steel housing. This type of construction was
adequate for current ratings of 225–600 A.

Bus-bars or Buses
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• The bus-bars were also covered with insulation for safety and to permit closer spacing of bars of
opposite polarity in order to achieve lower reactance and voltage drop.

Central Role of Computer in DSP – System Approach
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• Distribution system planners use computer based tools to perform the tedious calculations
necessary for system analysis.
• It has only been in the past few years that technology has provided the means for planners to truly
take a system approach to the total design and analysis.
• The system approach using computer tools starts by examining the types of information
required and its sources.
• The methodology used is that the information generated using the tool which are actually final
decisions and information are passed from one stage of the design process to another.
• It is thus noted that the human engineer must evaluate the information generated and add his or
her input. Finally, the results must be displayed for use and stored for later reference.
• The system approach seeks to automate as much of the process as possible, ensuring in the process
that the various transformations of information are made as efficiently as possible

Central Role of Computer in DSP – System Approach
• A database management system (DBMS) that stores, retrieves, and modifies various data on
distribution systems
• In addition to the database management program and the
network analysis programs.
• One such new tool is network editor.
• The features of the network editor may include network
objects, for example, feeder line sections, secondary line
sections, distribution transformers, or variable or fixed
capacitors, control mechanisms, and command functions.

Distribution System Automation
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• The main purpose of an electric power system is to efficiently generate, transmit, and distribute
electric energy.
• The operations involved aims at geographically and functionally complex monitoring and control
systems.
• The supervisory control and data acquisition (SCADA) system involves generation and
transmission systems.
• The distribution automation and control (DAC) system oversees the distribution system,
including connected load.
• Automatic monitoring and control features have long been a part of the SCADA system.
• The term distribution automation has a very broad meaning, and additional applications are added
every day.
• To some people, it may mean a communication system at the distribution level that can control
customer load and can reduce peak-load generation through load management.

Distribution System Automation
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• To others, the distribution automation may mean an unattended distribution substation that
could be considered attended through the use of an on-site microprocessor.
• The microprocessor, located at a distribution substation, can continuously monitor the system,
make operating decisions, issue commands, and report any change in status to the distribution
dispatch center, store it on-site for later use, or forget it, depending on the need of the utility

Thank You
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Distribution System Planning and Design

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